The lactase gene in Africa: Do you take milk?

20/1/07. By Harriet Coles

Convergent evolution in human populations is revealed by the recent discovery in East Africans of new variants of the lactase gene.

The genetic basis of lactose tolerance, which allows dairy consumption into adulthood, is well understood in European populations. A new study shows that the same dietary behaviour in East African communities is served by distinct genetic variants of the lactase gene that have evolved independently of the European alleles.

Most adults in the world are intolerant to milk. After weaning, the gene for the enzyme lactase, which breaks down the major sugar in milk – lactose – into easily absorbable sugars, is switched off. In regions of the world with a strong pastoral tradition, however, lactose tolerance commonly persists into adulthood. Most adults in these regions carry a regulatory
variant of the lactase gene that allows lactase production throughout life.

This lactase-persistence allele is found in more than 90 per cent of Danes and Swedes, and 50 per cent of Spanish and French, but is rare in non-pastoral communities such as the Chinese (1 per cent). Oddly, it also occurs at a low frequency in the pastoral communities of East Africa, even though many adults are lactose tolerant.

An international team of researchers, including Panos Deloukas at the Wellcome Trust Sanger Institute, has now found that lactose tolerance in East African adults is served by three newly discovered variants of the lactase gene. As in the European allele, the causative variants are single nucleotide polymorphisms in the control region of the gene.

And as in Europe, the alleles seem to have spread rapidly to a high frequency at the time that a pastoral lifestyle was being adopted. However, these distinct variants arose independently of, and several thousand years later than, the European allele.